Small-Scale Insights into Seismic-Scale Slumps:
A Comparison of Slump Features
from the Waitemata Basin, New Zealand,
and the Møre Basin, Off-Shore Norway
S. Bull and J. Cartwright
Abstract The results of a comparison between two different submarine mass
wasting events, using differing methods of investigation are presented in this study.
Remotely sensed geophysical data types, such as 3D seismic, can be used to image
and analyze large-scale examples. The strengths of such techniques lie in the ability
to image large areas (i.e. 10’s of square kilometers), allowing consideration of the
broad-scale architecture of the units. In contrast, field-outcrop studies allow more
detailed analysis of geological features, down to a millimeter scale. Two slumps
have been compared; one exposed in outcrop, the other imaged using 3D seismic
data. The outcrop example comes from the Miocene of the Waitemata Basin, northern New Zealand, and the seismic example comes from the Pliocene of the Møre
Basin, off-shore Norway, and occurs on a scale some 100 times greater that the
field outcrop example. Despite this, the two examples exhibit similarities in their
gross architecture, with each showing a bi-partite anatomy, well developed slump
folds and geometrically similar basal shear surfaces. The results of the comparison
emphasize the complexity of large-scale events, and illustrate the potential value of
combining field outcrop data with geophysical data types.
Keywords Mass wasting • 3D seismic • outcrop studies • New Zealand • Norway
1
Introduction
Historically, much insight into the dynamic evolution and emplacement mechanisms
of submarine mass wasting events has come from field-based studies of ancient outcrops,
which typically allow fine-scale, two-dimensional analysis (e.g. Farrell 1984).
S. Bull () and J. Cartwright
Cardiff University, 3D Lab, School of Earth, Ocean and Planetary Sciences, Main Building,
Park Place Cardiff CF10 3YE, UK
e-mail: SBULL@talisman-energy.com
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
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S. Bull and J. Cartwright
Fig. 1 (a): Location of the field outcrop area. Shaded area indicates extent of the Waitemata
Basin (after Strachan 2002). (b): Photomontage of part of the outcrop. Note the sequence of folds
defined by coherently-deformed coarse sandstone beds, and the segregation of the slump into two
structurally and sedimentologically distinct units, termed the upper and lower units
An increasingly common technique is the analysis of remotely sensed, geophysical
data such as three-dimensional (3D) reflection seismic (e.g. Frey Martinez et al.
2005). The advantages of using such data lie in the ability to perform complete
volumetric and geometrical analysis of both external and internal elements of sedimentary units over large areas. A drawback is the limitation imposed by seismic
resolution, typically in the region of c. 10 m vertically. In order to address this scale
problem, observations from an outcrop study of the Miocene-age Little Manly
Slump from New Zealand (Fig. 1a), chosen because of the excellent 2D exposure
and its compact scale, have been compared with a much larger slump from the
Pliocene of the Møre Basin, offshore Norway (Fig. 2a). This slump, referred to as
‘Slump W’ (after Lawrence and Cartwright 2009) has been chosen because it is
well imaged by high quality seismic data, and occurs on a scale some 100 times
greater than the field outcrop example.
The aim of this paper is to compare observations made in the field to the seismic
example in order to gain insight into processes involved in the dynamic development and emplacement of large-scale mass wasting events, beyond that which is
evident from comparatively low-resolution geophysical data types.
Small-Scale Insights into Seismic-Scale Slumps
259
Fig. 2 (a): Map showing the location of the seismically imaged Slump W. The extent to which
Slump W has currently been mapped is indicated by the dark grey shading (after Lawrence and
Cartwright 2009). Arrows indicate transport direction. FSE: Faeroe-Shetland Escarpment. Outline of
Storegga Slide Complex shown. (b): Outline of 3D seismic survey with time structure map of
Horizon X, which divides Slump W into two distinct seismic facies packages. The arcuate lineations
(dashed line) are interpreted to be the result of compression. Arrow indicates transport direction. (c):
Representative seismic dip-profile through Slump W. Note upper and lower packages. Location
shown in (b). (d): Zoom in of seismic profile showing two slump folds. Note disharmonic, stacked
nature of the reflections within the folds and brittle discontinuities. Location shown in (c)
2
Dataset and Methodology
The data used in this study consists of a set of field observations and photographs
from the Little Manly Slump, taken during fieldwork conducted in 2007, and a modern, industrially acquired 3D reflection seismic survey which images Slump W. An
integrated 3D seismic interpretation approach has been applied (Cartwright 2007),
and the resolution of the seismic data has been calculated as c. 9 m vertically (maximum vertical resolution = l/4; Sheriff and Geldart 1983) using an assumed average
seismic velocity of 2,000–2,100 ms−1 for the interval of interest. Horizontal resolution
is estimated at 45 m (taken to approximate the dominant seismic wavelength).
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Geological Settings
Little Manly Slump: the field study area is located c. 40 km north of Auckland on
the Whangaparaoa Peninsula, where the slump is exposed for c. 600 m at Little
Manly Beach, in 8–15 m high cliffs made accessible by wavecut platforms. The
Little Manly Slump occurs in the Miocene infill of the Waitemata Basin (Fig. 1a),
whose initiation is thought to be linked to the development of a new convergent
plate-tectonic regime represented in the present day by the Alpine Fault/Hakurangi
Trough plate boundary (Ballance et al. 1982). The basin fill reflects initial shallow
marine conditions followed by rapid deepening, with the basin receiving mostly
turbiditic clastic sediments (Ricketts et al. 1989). The Little Manly Slump is interpreted to have developed in an outer fan-setting, translating downslope to the SW
(Strachan 2002). The slump incorporates three main lithologies: medium- and
coarse-grained turbidite sandstones, and a fine-grained mudstone interpreted as
background hemipelagic sedimentation (Strachan 2002).
Slump W is found in the Møre basin on the passive Norwegian continental margin. Initiating in the Cretaceous, the basin underwent continuous, thermally driven
subsidence (Brekke 2000), and was filled by fine-grained hemipelagic oozes of the
Palaeogene age Brygge Formation and Miocene – earliest Pliocene age Kai
Formations (Evans et al. 2002). The Plio-Pleistocene Naust Formation, within which
Slump W occurs, comprises of contourites, hemipelagites and glacigenic sediments
(Evans et al. 2002). Slump W is believed to have developed between 1.8 and 4 Ma
and is confined against the volcanic palaeo-high of the Faroe-Shetland Escarpment
(Lawrence and Cartwright 2009). Lawrence and Cartwright (2009) concluded that
limited translation of the slump mass has occurred (estimated at 2–3 km), with a
transport direction from east to west.
4
Little Manly Slump Description
The Little Manly slump has recently been the subject of previous studies (e.g.
Strachan 2002, 2008) and as such the dynamic evolution and emplacement mechanisms are well constrained. The Little Manly Slump comprises a c. 2.5 m thick unit
which is divisible into distinct upper and lower packages, the lower featuring many
contorted beds and a high degree of lateral variation in the style and intensity of
deformation, and the upper consisting of a massive sand (Fig. 1b). The lower unit
exhibits a variety of ductile, brittle, compressional and extensional structures,
including folds of various styles and sizes and intra-slump to slump-scale normal
and thrust faults. The upper unit varies in thickness between 0.01–2 m and exhibits
occasional large-scale cross bedding, broad undulations and clasts in the lower part.
The top surface of the upper unit is sharp and overlain conformably by further sandrich units. Segregation of the slump into two depositional units is interpreted to be
the result of progressive deformation of the lower unit during slump translation
Small-Scale Insights into Seismic-Scale Slumps
261
with co-eval development and deposition of the upper unit, thought to be a turbidite
sourced from the deforming mass below (Strachan 2008).
The structural style of the lower unit is dominated by ductile folding, exhibiting
a variety of fold styles and scales (Fig. 2b). A large part of the outcrop is dominated
by a spectacular sequence of folds defined by c. 0.2 m thick, coarse sandstone beds
which appear to have been deformed in a relatively competent manner, with significant continuity and uniform bed thickness exhibited (Fig 1b), and faults which
affect the thickness of the beds present at the fold hinges (Strachan 2002).
An interesting observation is the apparently ‘less competent’ behavior of other
lithologies also present within the folds. Figure 3a shows an example of a coarse
sandstone fold which has been truncated by the upper unit. In the ‘core’ of the fold,
above the basal shear surface, a ‘plug’ of coarse-grained material is present (indicated
in Fig. 3a), along with an intensely deformed mudstone bed which appears to have
partially mixed with the coarse-grained material. The mudstone bed exhibits
intense folding and a lenticular ‘pinch and swell’ morphology with occasional
complete bed rupture resulting in isolated inclusions of the mudstone within the
coarse-grained material (Fig. 3a).
A separate fold also features similar competency contrasts and variation of
deformation. Figure 3b shows the fold which is defined by two competent sandstone
beds. Again within the ‘core’ of the fold, the medium-coarse grained lithology and
the mudstone have deformed in a relatively less-competent manner (Fig. 3b).
a
b
Fig. 3 Example of folds from lower unit of Little Manly Slump. (a): Fold formed by coarse
sandstone bed. The ‘core’ of the fold is occupied by a plug of medium-coarse grained material
around which a thin, fine-grained mudstone has deformed in an intense, highly ductile manner.
(b): Example of a recumbent fold defined by coarse sandstone beds. Between fold limbs, the
medium-coarse grained lithology and fine-grained mud exhibit competency contrasts and intense
ductile deformation. Locations shown in Fig. 1b. ‘E’ indicates east
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S. Bull and J. Cartwright
Here too the mudstone bed is intensely folded and exhibits variations in thickness
expressed as an overall ‘pinch and swell’ morphology, within the more homogenous matrix of the medium-coarse grained material. Where the fold limbs are very
tightly folded both the medium-coarse grained lithology and mudstone are present
with the mud once again having deformed in a ductile sense within the coarser
material (Fig. 3b).
These features were attributed by Strachan (2008) to shearing resulting from deformation of surrounding material and competency contrasts between the varying lithologies. Gregory (1969) remarked that some of the deformational fabrics exhibited by the
lower unit of the Little Manly Slump attested to the ‘…plasticity, verging on fluidity
of sediments during deformation…’ and considered that ‘thixotrophic mixtures of
mud, silt and sand have flowed or been squeezed into their present positions.’
The basal shear surface occupies a single discrete horizon (Figs. 1b and 3),
and occurs for the most part above a coarse sandstone bed but ramps up and down
locally, by up to 0.5 m. Occasionally, beds below the basal shear surface are
disrupted, fragmented or rotated, interpreted to be the result of shear from the
overlying slump, and as such could be considered to be part of the slump (Strachan
2002). Ptygmatically folded dykes are present beneath the sandstone bed which
predominantly underlies the basal shear surface of the slump, where they cross-cut
a fine-grained, laminated mudstone unit and are aligned parallel to the slumping
direction (Strachan 2008). The dykes are interpreted to be the result of contemporaneous mobilization of underlying sandstones related to slump-induced loading
and lateral shearing (Strachan 2008). The basal shear surface is thought to originally have been a low stress, clay rich horizon, possibly of volcaniclastic origin
(Strachan 2002).
5
Slump W Description
Slump W covers an area of over 21,000 km2 (Fig. 2a) and averages 200–250 m in
thickness. Two distinct seismic facies packages can be identified, separated by a
continuous seismic reflection (labeled Horizon X on Fig. 2c) which can be traced
throughout the 3D survey area (Fig. 2b, c). The lower unit is characterized by discontinuous, contorted reflections bound at the base by a relatively flat-lying, bed
parallel reflection (labeled Horizon Y in Fig. 2c). The interval between Horizon
X–Horizon Y varies from a maximum thickness of 100 m to a minimum of <5 m
and comprises a number of partially continuous, high amplitude reflections (Fig.
2c). The amplitude, continuity and deformation of the various reflections varies
considerably across the survey area. Some zones appear highly disaggregated while
in other places reflections continuous over a distance of up to 1 km define recognizable deformational features such as folds and thrust faults (labeled on Fig. 2c).
Horizon X is interpreted as representing the upper geological boundary of the lower
unit of Slump W, and is highly-undulatory in character (Fig. 2b). A time structure
map of Horizon X shows that the undulations in seismic profile correspond to a
Small-Scale Insights into Seismic-Scale Slumps
263
series of parallel to sub-parallel, linear to arcuate ridges continuous across the
survey area (Fig. 2b). The most likely origin of these structures is compression,
resulting from close proximity to the confining distal margin of the slump
(Lawrence and Cartwright in press).
The upper unit, whose lower boundary is Horizon X, also shows internal variation
in the amplitude, continuity and deformation of internal reflections (Fig. 2c). The
upper unit is of generally lower seismic amplitude than the lower unit and contains
much fewer and less well defined reflections (Fig. 2c). This characteristic is interpreted to represent a package comprising of fewer lithologically contrasting beds
which have experienced a lesser degree of deformation than the lower unit. The top
surface of the upper unit (labeled ‘Horizon Z’ on Fig. 2c) is mildly undulatory in
character, exhibiting up to 10 m in vertical relief (Fig. 2c).
The internal deformational character of the lower unit varies across the survey
area, and features evidence of a variety of ductile and brittle deformational styles
(Fig. 2c). The predominant deformational style is reminiscent of a series of ductile,
predominantly upright folds with varying internal character (Fig. 2c). Two of the
structures in the lower unit have a similar gross morphology to some of the folds
observed at Little Manly (Fig. 2d). The structures have been interpreted as slump
folds and are labelled ‘Fold 1’ and ‘Fold 2’ in Fig. 2d. Horizon X forms the top of
the folds, which exhibit some 100 m of vertical relief and have outward dipping
flanks which slope at 7° (Fig. 2d). Inside the folds, continuous, stacked reflections
have been deformed into packages of convex-upward, disharmonic folds which also
appear to be affected by internal brittle discontinuities (Fig. 2d). The folds are
elongate in planform, measuring up to 1.75 km in length, 1 km across and are c.
250 m high. Above Horizon X, the folds are infilled by the upper unit (Fig. 2d).
Horizon Y is interpreted as being a close approximation to the basal detachment
surface of Slump W (Fig. 2c), as it is the first relatively continuous, un-distorted
reflection occurring below the deformed slump interval. Below Horizon Y, small,
localized deformational zones are observed (Note that reflections here are not completely flat-lying due to the influence of an underlying faulted unit; Fig. 2d). The
package of reflections directly underlying the lower unit of Slump W, including
Horizon Y, exhibit localized, mild disruption, small offsets and lateral changes in
amplitude (Fig. 2c, d), which are here attributed to deformation caused by the
emplacement of the overlying slump.
6
Discussion
The most striking observation made from the Little Manly Slump is the complexity
and intensity of deformation which is evident beyond the larger-scale gross geometry of the unit as a whole. Considering Slump W, which provides an example of a
slump on a scale some 100 times larger than the Little Manly example, the question
is raised of what features or evidence for similar processes may be present in Slump
W, but simply not captured by the seismic image? To help address this question,
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c
b
a
e
d
Fig. 4 (a): Seismic profile (originally shown in Fig. 2d) from Slump W populated with field
photographs from Little Manly to illustrate potential complexity of such a large-scale depositional
unit. (b): Infilling of lower unit by upper unit; (c): Slump folds with complex competency contrasts and highly ductile deformation; (d): Clastic dykes below the basal shear surface; and
(e): disruption of beds below the basal shear surface. ‘E’ indicates east
Fig. 4 shows a zoomed in cross-sectional view of some of the key structural
elements described from Slump W, populated with field photographs of comparable elements from Little Manly. Despite large differences in the sizes of the slumps,
both examples show remarkably similar gross-scale architecture and internal
elements including: (1) a bi-partite upper and lower unit division,; (2) internal folding
comprised by various lithological units of differing competencies and (3) basal
shear surfaces which are generally flat-lying and in places show localized disruption of units immediately below. These characteristics introduce the possibility that
the Little Manly Slump may represent a useful analogue to Slump W and could be
used to restore information relating to deformational elements and emplacement
processes which are not resolved by the seismic data.
A common characteristic of both slump examples is the clear division into upper
and lower units. In both cases, an intensely deformed lower unit, dominated by
ductile deformation, is overlain by a less complex unit which infills the topography
of the lower unit (Figs. 1b and 2c). For Little Manly, a co-eval scenario is invoked
whereby a bi-partite ‘couplet’ comprising a lower unit deforming ductily and a more
Small-Scale Insights into Seismic-Scale Slumps
265
fluid-like upper unit, moved downslope and were deposited together (Strachan
2008). In the case of Slump W, it is possible that a similar model could be accountable, although evidence is difficult to pinpoint given the current dataset. However,
by drawing upon the field example, some inferences can be made.
That Slump W features a lower unit dominated by the predominantly ductile
deformation of coherent lithological units such that they were able to form upright
slump folds, suggests that like Little Manly, the lower unit of Slump W was
emplaced by way of a typical, progressively deforming slump mechanism
(Martinsen 1994). The upper unit of Slump W is interpreted to consist of a less
deformed, more uniform lithology than the lower unit. While evidence which supports the interpreted co-eval scenario for Little Manly, such as sedimentary structures and inclusions from the lower unit cannot be identified from the seismic data,
the upper units from both slumps can be likened on a broad scale. While there is
scope to invoke further scenarios for the development of Slump W, the co-eval
model borrowed from Little Manly illustrates how slumps studied in the field can
be used as a basis for comparison to seismically imaged examples, and to direct
further analysis.
Evidence for intense shearing and possible mobilization of less competent
lithologies was intimately linked to the well developed folds in the competent
coarse sandstone beds at Little Manly, occurring within and around the folds and
between beds of differing competency. The folds from Slump W shown in Fig. 4
are clearly comprised of many layers of deformed material, but from the seismic
record, it is not clear which lithologies are behaving relatively competently and
which are behaving relatively incompetently – only the gross configuration is
apparent. Although the gross geometry of the folds suggests a ductile but ‘competent’ nature of deformation, it is clear from Little Manly that some lithologies could
be behaving in a potentially fluid manner, and that small scale liquefaction could
be involved in the development and final geometry.
Sheared dykes and slump-scale faults were observed to affect the basal shear
surface at Little Manly, with dykes of c. 0.1 m in height and 0.02 m across (Fig. 4d).
The basal shear surface of Slump W was approximated to a single moderateamplitude trough in the seismic record (Horizon Y), with no evidence of shear or
further deformation observed other than local undulation and mild disruption
(Fig. 2c). Given the required conditions, i.e. presence of an over-pressured sand
unit beneath Slump W, it is possible that the basal shear surface of Slump W may
also be associated with clastic dykes, which due to their scale and vertical geometry
would not be imaged by the seismic data. The flat lying, concordant nature of the
Little Manly Slump basal detachment indicates that the slump moved over flat lying
bedding (Strachan 2008), and the same is taken to be true for Slump W. At Little
Manly, disruption and rotation of beds in close underlying proximity to the basal
shear surface was interpreted to result from lateral shear and loading by the overlying
slump, and possibly representative of down-cutting and entrainment of underlying
beds by the slump mass (Strachan 2002, 2008). This can also be applicable to
Slump W as evidenced by amplitude variations and localized disruption of Horizon
Y and of reflections immediately underlying it (Fig. 4e).
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S. Bull and J. Cartwright
Conclusions
Comparison of several key characteristics of two slumps, one studied in outcrop
and one imaged by modern 3D seismic data, has served to illustrate the level of
complexity that may be present in large-scale examples, but is beyond the resolution limitations of modern geophysical data. The present study has shown how the
application of field-scale observations to increasingly common geophysical studies
can help to restore sub-resolution information and provide a basis for the development
of depositional models, ultimately leading to an improved understanding of the
depositional units resulting from submarine mass wasting events.
Acknowledgments The authors wish to thank Statoil Hydro (Andreas Helsem) for provision of
data and Schlumberger for seismic interpretation software. Rob Evans helped in the preparation
of the manuscript, and Tom Praeger provided assistance in the field and discussion of the topics
presented in this paper. Lorna Strachan and Aggeliki Georgiopoulou are gratefully acknowledged
for their reviews which significantly improved the structure and quality of the paper.
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